The invention relates to a hand tool, particularly for the accurate insertion of screws or the like, having an electric drive motor arranged in a housing for the operation of a tool bit such as a screwdriver bit, and an axially adjustable depth stop by means of which the drive motor can be switched off automatically when said depth stop contacts a workpiece, and having a switching mechanism connected to the drive motor, into the electric circuit of which are installed a switch for switching the drive motor on and off, a first contact element for reversing the polarity of the drive motor for clockwise and counter-clockwise rotation and a second contact element for switching off the drive motor by means of the depth stop.
A hand tool of this nature is disclosed by EP-0 591 096 A1. In order to be able bring about a reversal of the direction of rotation of the drive motor in this device, it is not only necessary to actuate the first contact element for polarity reversal of the drive motor, but rather a disengageable transfer element must also be moved by hand. In this case the transfer element located between the depth stop and the first contact element and which must be manipulated from the outside is comprised of a ring that is rotatable about the axis of the housing. Joined to the ring is a control cam with various axial extensions. Rotating the ring thus creates a free space for the first contact element so that it is no longer actuated, even though the depth stop is still in contact with the workpiece, and the drive motor can be switched into the opposite direction of rotation.
Aside from the considerable constructional expense required in this embodiment in order to make it possible to restart the drive motor when the depth stop is in contact, this operation can only be brought about with difficulty. The depth stop is under initial tension when in contact with the workpiece so that this bearing pressure must first be relieved in order to be able to then turn the ring which is bound to the depth stop by friction. This sometimes requires a difficult coordination of movements, since the tool clamped into the hand tool should or must often not be removed from a component part, such as a screw, to be manipulated. However, if the device is removed from the workpiece in order to turn the ring manually, the hand tool must be placed back again so that the component part can be screwed out. It is thus not possible to obtain optimum performance with the known device; on the contrary, operations that require the drive motor to drive the tool in both directions of rotation require a considerable amount of time.
The object of the invention is therefore to devise a hand tool of the type described above, the drive motor of which can be switched from clockwise to counter-clockwise rotation without the necessity of mechanical means and without the necessity of any manual manipulation of adjustment elements. Rather, the reversal of polarity of the drive motor should make it possible to automatically neutralize the locked position brought about by the depth stop by means of the second contact element so that screws can be driven in and out with no loss of time and without having to remove the hand tool. The constructional expense required to bring this about should be very low and almost no additional space should be required in the tool for the accommodation of the components needed for this purpose. Rather, working with such a device should be made easier to a considerable degree.